Certain novolak resins are known to those skilled in the art. These novolaks are conventionally prepared by condensing phenol and formaldehyde in the presence of an acid catalyst. Typically these known novolak resins are prepared with less than one mole of formaldehyde per mole of phenol. Generally, the most common starting materials are phenol and aqueous formaldehyde. These starting materials are selected because of their good reactivity, availability and low cost. The acid catalyst is either an inorganic type such as sulfuric acid, hydrochloric acid, phosphoric acid, or other types known in the art. The acid may also be an organic acid such as p-toluenesulfonic acid, methanesulfonic acid or oxalic acid. Certain catalysts are chosen for their reactivity or their influence on the properties, such as color.
These known novolak resins, upon heating, do not harden to an insoluble or infusible condition but remain organic solvent soluble and fusible. They can be hardened or cured by the addition of a curing agent, such as hexamethylenetetramine or trioxane, and further heating.
In a typical procedure to prepare a conventional novolak, the phenol and catalyst are intimately mixed. The formaldehyde is added either by initially mixing with the phenol and catalyst, portionwise, or continuously over a defined time. Because of practical considerations, the formaldehyde is usually added portionwise or continuously. The reaction generates heat, so cooling is often required. Once the initial exotherm is completed, the mixture is heated to complete the reaction of the two components. The novolak resin usually separates from the water layer and can be isolated at this point. Alternatively, the reaction mixture can be dehydrated under heat and vacuum to isolate the novolak resin. The novolak resin is a solid at room temperature.
The conditions of a typical procedure produce a novolak with various chain lengths, and the reaction of the formaldehyde with the phenol is usually random. This produces a novolak with varied substitution of the phenol ring. Depending on the reaction conditions and the type of phenol used, the molecular weight and softening temperature of the final resin may vary. Typically, the number average molecular weight, as determined by size exclusion chromatography, ranges from 300 to 2000 or higher.
Generally, the source of aldehyde used to prepare these known novolak resins is aqueous formaldehyde. Paraformaldehyde, trioxane and hexamethylenetetramine have also been used for this purpose.
Various phenols have been used to prepare these known novolaks. Such phenols include cresols, xylenols, biphenols, catechols, hydroquinones, naphthols and the like, including mixtures of the same. Certain phenols are chosen to impart specific properties, such as varying the softening temperature, water solubility or insolubility, color and melt viscosity.
Certain phenols used to prepare these known novolaks contain two or more adjacent hydroxyl groups. Such phenols include pyrogallol, catechol, tannic acid, gallic acid and the like. The phenols which have adjacent or vicinal hydroxyl groups, are known to be metal chelators. Metals such as iron, copper, manganese, vanadium, and others form complexes with these types of phenols (See: S. Yariv, W. Bodenheimer, and L. Heller, Israel J. Chem. 1964, 2(5), pp. 201-8; I. P. Mavani, C. R. Jejurkar, and P. K. Bhattacharya, J. Indian Chem. Soc. 1972, 49(5), pp. 469-74; R. Griesser and H. Sigel, Inorg. Chem. 1971, 10(10), pp. 2229-32; J. Zelinka and M. Bartusek, Collect. Czech Chem. Commun. 1971, 36(7), pp. 2628-37; G. F. Condike and A. E. Martell, J. Inorg. Nucl. Chem. 1969, 31(8), pp. 2455-66; H. Sigel, P. R. Huber, R. Griesser, and B. Prijs, Inorg. Chem. 1973, 12(5), pp. 1198-200; and M. A. Kessick, J. Polym. Sci., Part B, 1972, 10(7), pp. 527-30).
Uses of known novolak resins include coatings, adhesives, fiber bonding abrasives, laminates, foundry resins, friction materials, molding materials and wood composites. Certain known novolaks have been used as adhesion promoters for epoxy resins.
U.S. Pat. No. 3,304,276 (Faulkner and O'Neill) describes the preparation and use of metal-reactive polyhydric phenol-modified short oil and long oil alkyds that cure even at room temperature to form rust inhibiting, essentially non-blistering, durable finishes on unoxidized iron and steel surfaces. The polyhydric phenols include tannins, methyl gallate, propyl gallate, catechol and pyrogallol. These authors found that Fe/polyhydric phenolic chelate complexes are formed with the clean ferrous surface.
U.S. Pat. No. 4,340,716 (Hata et al.) and Japanese J5 6,149,716 teach that good coating compositions showing good adherence and good anticorrosive properties can be prepared by combining (A) a prepolymer of epoxy resin and polyhydroxyphenolic compound with (B) an organic hardener for epoxy resins. The prepolymer (A) is prepared by prereacting small amounts of polynuclear adjacent aromatic hydroxyl-containing compounds, including catechol and pyrogallol novolaks with epoxies. The reaction ratio of the epoxy resin and the polyhydric phenolic compound is 1.0/0.05 to 1.0/0.1 (mole/mole).
U.S. Pat. Nos. 4,560,732 and 4,530,947 (Kojo et al.) teach a laquer-type coating composition for metals that forms chelate bonds with the metal to provide strong adhesion to the metal. This solvated coating composition comprises the reaction product of epoxy resins and polynuclear polyhydroxyphenols, including catechol and pyrogallol novolaks. These reaction products have M.sub.n values of 12,000 to 31,000 and are prepared at ratios insuring that unreacted adjacent aromatic hydroxyls are available. These coatings are applied as lacquers and are unsuitable for making powder coatings because the dried out resins will not melt flow adequately.